"Tamoxifen and retinoid regulation of antiproliferative pathways in high risk human breast epithelium cells (HRBEC) and in breast cancer cells is being studied to characterize high risk cells and to develop new treatment strategies for the chemoprevention and treatment of breast cancer. Studies in progress have demonstrated that HRBEC are growth inhibited by all-trans- (AT) and 9-cis (9C) retinoic acid, and that this inhibition is due to blockade of cell cycle progression at G0G1 without stimulation of apoptosis. AT and 9C modulation of protein expression of the retinoic acid receptors RAR-alpha, RAR-beta, RAR-gamma, of transforming growth factor beta (TGF-beta), and cyclin D1, cdk4, and retinoblastoma proteins have been studied to define the mechanism of action of these retinoids. The findings indicate the antiproliferative effects on expression of retinoic acid receptors and cell cycle regulatory proteins in high risk breast epithelial cells are intermediate between normal breast epithelial cells and breast cancer cells, and thus may represent early changes in carcinogenesis. To further define the characteristics of high risk mammary epithelium, an intramural clinical trial is in progress to establish short-term mammary epithelial cell lines from women at high risk for breast cancer (including BRCA-1 positive). The growth, metabolic, and molecular properties of these cells will be examined. The effects of tamoxifen and retinoic acid on cell cycle progression, cell differentiation, and apoptosis of human breast cancer cells is also being studied to define the synergistic effect of these agents and to identify new targets for the treatment of breast cancer. Studies in progress have demonstrated that AT acts synergistically with tamoxifen to regulate multiple antiproliferative pathways in breast cancer cells, including a time-dependent early (day 4) stimulation of TGF-beta secretion and induction of cell differentiation, followed by a late (day 7) downregulation of the apoptotic protein bcl-2 and stimulation of apoptosis. Cell cycle progression was not synergistically regulated, indicating selective recruitment of the antiproliferative pathways. TGF-beta is an important mediator of TAM action on breast cancer cells, which suggested a potential role in the synergistic effects of AT/TAM in combination. These studies revealed that TGF-beta mediates the synergistic antiproliferative effects of TAM and AT in combination, and by a novel autocrine action of TGF-beta: combining with AT to downregulate bcl-2 protein expression and stimulate apoptosis. Studies are in progress to define the mechanism of AT/TGF-beta interaction. These findings also suggest several important targets (cell differentiation antigens,TGF-beta, bcl-2) which may be utilized for new treatment strategies for breast cancer. A second mediator of apoptosis in breast cancer cells, Fas, is also being examined. AT and gamma-interferon in combination have been found to induce Fas expression, and when combined with Fas-antibody, greatly enhance apoptosis of breast cancer cells in vitro. These observations are being developed in vitro and in vivo, and indicate a potentially important new treatment strategy for breast cancer."